Novell Del Mar College
CIS 306 - Managing NOVELL® Networks
 Instructor: Michael P. Harris
Networking Primer

6. Internetworking

As a business grows, it might need to split its network. Or, a business might need to connect two separate networks so that users on each can use resources on either. When a network is split (or when two networks with different addresses are connected), this results in an internetwork. An internetwork has subnetworks (network segments) that have different network addresses. Even a modest-sized business often has several subnetworks operating, each serving a specific portion of the organization.

Why might a business need subnetworks?

The most common reason for segmenting a network is to preserve excellent network performance. On even the fastest and most efficient network, if the network has too many users (devices that need to transmit), the transmission media can become so busy that devices have to wait an unacceptable time to transmit. When this happens, users begin to notice delays when they try to save or open files or perform other operations.

When you segment a network, you give each subnetwork its own network address. This results in two separate transmission media segments, which can be used simultaneously. Each of the two segments will have only half the users of the original network. Thus, you double network performance (on some networks, performance can more than double because on an overloaded network, the overhead required to manage transmission collisions takes a much larger percentage of bandwidth than on a modestly busy network).

Networks are also segmented to enhance data security and to minimize the effect of equipment failure on any part of the network.

Internetworking includes everything from connecting two small workgroup networks, each with perhaps two or three workstations, to connecting thousands of computers—from notebook computers to mainframes—on tens to hundreds of individual segments in a worldwide organization.

Internetworking Devices: Bridges and Routers

Bridges and routers are the devices used to interconnect subnetworks. They can be primarily hardware based or primarily software based.

Software-based routers and bridges can be part of a server's operating system or can at least run in the server with the operating system. Software-based bridges and routers can also be installed on standard computers to create dedicated, standalone devices. For example, IntranetWare MultiProtocol Router software is a family of software-based routing products that can be installed on an IntranetWare, NetWare 4, or NetWare 3™ server or on a standalone PC.

To understand internetworking, it is not essential that you understand all the technical differences between a bridge and router. In fact, without some study, this can be a confusing area. For example, if you read about IntranetWare MultiProtocol Routers, you will find that these routers also perform what is called source-route bridging.

However, without a basic understanding of bridging and routing technology (and related terminology), you will find it difficult to understand the capabilities of some products and the reasons such capabilities are useful or important. Please keep in mind throughout the following discussion that bridges and routers have one important thing in common: They both allow the transfer of data packets (frames) between subnetworks with different network addresses.

Bridges

A bridge operates at the data-link layer (layer two) of the OSI model. A bridge acts as an address filter; it relays data between subnetworks (with different addresses) based on information contained at the media access control level.

Simple bridges are used to connect networks that use the same physical-layer protocol and the same MAC and logical link protocols (OSI layers one and two). Simple bridges are not capable of translating between different protocols.

Other types of bridges, such as translational bridges, can connect networks that use different layer-one and MAC-level protocols; they are capable of translating, then relaying, frames.

After a physical connection is made (at OSI layer one), a bridge receives all frames from each of the subnetworks it connects and checks the network address of each received frame. The network address is contained in the MAC header. When a bridge receives a frame from one subnetwork that is addressed to a workstation on another subnetwork, it passes the frame to the intended subnetwork. Figure 19 illustrates, in a general fashion, how a bridge relays frames between subnetworks.

Figure 19: Internetworking through a bridge

A bridge assumes that all communication protocols used above the data-link layer at which it operates (OSI layers three through seven) are the same on both sides of the communication link. Of course, this must be true, or there must be translation between unlike protocols at layers three through seven for the receiving computer to be able to interpret the transferred data.

Spanning Trees and Source-Route Bridging

There are two terms connected with bridging that will be useful to understand: spanning trees and source-route bridging.

Spanning trees prevent problems resulting from the interconnection of multiple networks by means of parallel transmission paths. In various bridging circumstances, it is possible to have multiple transmission routes between computers on different networks. If multiple transmission routes exist, unless there is an efficient method for specifying only one route, it is possible to have an endless duplication and expansion of routing errors that will saturate the network with useless transmissions, quickly disabling it. Spanning trees are the method used to specify one, and only one, transmission route.

Source-route bridging is a means of determining the path used to transfer data from one workstation to another. Workstations that use source routing participate in route discovery and specify the route to be used for each transmitted packet. Source-route bridges merely carry out the routing instructions placed into each data packet when the packet is assembled by the sending workstation—hence the name "source routing." In discussions of bridging and routing, do not be confused by the term "source routing." Though it includes the term "routing," it is a part of bridging technology. Source-route bridging is important because it is a bridge-routing method used on IBM Token-Ring networks.

You should understand that bridging technologies and routing methods can be combined in various ways. For example, there is an IEEE specification for a source-route transparent bridge, a bridging scheme that merges source-route bridging and transparent bridging in one device.

From this simple discussion of bridging, one thing should be apparent: When choosing internetworking products, it is important to select those that support the various bridging methods—products such as IntranetWare MultiProtocol Router. (For further details, see the IntranetWare MultiProtocol Router 3.1 product section.)

Routers

Routers function at the network layer (layer 3) of the OSI model (one layer above bridges). To communicate, routers must use the same network-layer protocol. And, of course, the sending and receiving workstations on different networks must either share identical protocols at all OSI layers above layer three, or there must be necessary protocol translation at these layers.

Like some bridges, routers can allow the transfer of data between networks that use different protocols at OSI layers one and two (the physical layer and the data-link layer, which includes sublayers for media access control and logical link control). Routers can receive, reformat, and retransmit data packets assembled by different layer-one and layer-two protocols. Different routers are built to manage different protocol sets. Figure 20 illustrates how a router transfers data packets.

Figure 20: Internetworking through a router

NetWare Internetworking Protocols

Before we conclude with a discussion of host connection, wide area networking technologies, and global networking, you should understand a little more about native NetWare protocols that play a role in NetWare internetworking. Figure 21 shows in greater detail how NetWare protocols fit into the OSI model.

Figure 21: Where NetWare protocols fit in the OSI model

Each of the native NetWare protocols shown in Figure 21 plays a role in NetWare internetworking, either directly or indirectly.

IPX: The Network Layer Protocol

In conjunction with industry-standard media access control protocols, the NetWare IPX™ protocol provides the NetWare addressing mechanism that delivers communication packets to their destination. IPX works with all important MAC standards. As you can see from Figure 21, IPX operates at the network layer of the OSI model.

In a NetWare environment, internetwork packet routing is accomplished at the network layer. Thus, IPX is the NetWare protocol that addresses and routes packets between internetworked computers.

IPX bases its routing decisions on the address fields in its packet header (provided by the MAC protocol) and on the information it receives from other NetWare protocols. For example, IPX uses information supplied by either the RIP or NLSP™ protocols to forward packets to the destination computer or to the next router. IPX also uses SAP.

RIP and NLSP: The Routing Protocols

NetWare routers use one of two routing protocols, RIP or NLSP, to exchange routing information with neighboring routers.

RIP: A Distance-Vector Routing Protocol

The NetWare RIP is a distance-vector protocol. Distance-vector routing protocols are the traditional method used for router communications.

In an internetwork using distance-vector routing, routers periodically determine if the internetwork configuration has changed. They also periodically broadcast packets to their immediate neighbors; these packets contain all information they currently have about the internetwork's topology.

After receiving any information, distance-vector routers consolidate the information and pass summarized data along to other routers, servers, and end devices, such as printers and workstations. Through this periodic checking and broadcasting, which is performed at regular intervals regardless of whether the internetwork has changed, all routers are kept updated with correct internetwork addresses for all computers and other connected devices, as well as with the best route for transferring data between any two devices.

Because RIP is a distance-vector protocol, NetWare routers that use RIP work in the way described above, performing periodic checking and information exchange and updating their routing tables with any new information.

RIP is one of a number of well-known distance-vector routing protocols. Examples of other such protocols include IP RIP and Cisco IGRP, part of the IP protocol suite, and RTMP, part of the AppleTalk protocol suite.

NLSP: A Link-State Routing Protocol

The NetWare Link Services Protocol is a link-state routing protocol. This type of protocol derives its name from the fact that link-state routers track the status of other routers and links.

Link-state protocols, a relatively recent development, adapt more quickly to network topology changes than do distance-vector protocols. Thus, they are better than distance-vector protocols for managing internetworking on large, complex internetworks.

In an internetwork that uses a link-state routing protocol, each router or server provides information about itself and its immediate neighbors to every reachable router in a routing area. Each router's map includes all the area's routers and servers, the links connecting them, and the operational status of each router and link. However, each router builds its own routing map rather than relying on secondhand summaries, as do distance-vector routers. Also, routing transmissions are made only when the internetwork changes, not at predefined intervals. Thus, networks using link-state routing are not burdened by unnecessary routing traffic.

Because NLSP works as explained above, it significantly reduces the communication overhead required for routing. NLSP can significantly improve network performance because it frees resources to be used for transferring data packets rather than routing information. NLSP is particularly efficient for wide area network routing, where available communication bandwidth is ordinarily limited.

Examples of other link-state protocols include the Open Shortest Path First protocol, part of the TCP/IP protocol suite, and the Intermediate System-to-Intermediate System protocol, a router-to-router protocol that is part of the OSI suite.

As a matter of note, various link-state and distance-vector routing protocols can coexist on the same NetWare internetwork and even in the same IntranetWare MultiProtocol Router. Furthermore, individual routers can be configured to accept or to reject individual protocols.

SAP: Service Advertising Protocol

The Service Advertising Protocol is similar in concept to RIP. Just as RIP enables routers to exchange routing information, SAP enables networked devices, such as network servers and routers, to exchange information about available network services.

Servers and routers use SAP to advertise their services and network addresses. SAP enables network devices to constantly correct their information about which network services are available. While servers are running, they use SAP to inform the rest of the network of the services they offer. When a server goes down, it uses SAP to inform the network that its services are no longer available.

Routers gather service information and share it with other routers. Workstations use the information made available through SAP to obtain the network addresses of servers that offer the services they need.

NCP: NetWare Core Protocol

The NetWare Core Protocol is a set of service protocols that a server's operating system follows to accept and respond to service requests.

NCP does not play a direct role in routing. However, it does provide session control and packet-level error checking between NetWare workstations and routers.

SPX: Sequenced Packet Exchange

SPX is a transport-layer protocol. Standards at this OSI layer provide for the reliability of the end-to-end communication link. Accordingly, SPX provides guaranteed packet delivery and packet sequencing.

Like NCP, SPX does not play a direct role in routing. SPX is connected with internetworking only in that it guarantees delivery of all routed packets.

Gateways

In contrast to bridges and routers, which function at only one layer of the OSI model, a gateway translates protocols at more than one OSI layer. Therefore, a gateway is used to interconnect computer systems that have different architectures and that therefore use different communication protocols at several OSI layers.

A gateway may connect dissimilar systems on the same network or on different networks (thus, using a gateway does not necessarily involve internetworking). For example, a gateway might translate protocols at several different OSI layers to allow transparent communications between NetWare IPX-based systems and systems based on TCP/IP, System Network Architecture (SNA), or AppleTalk. Figure 22 illustrates how a gateway is used to translate protocols to enable communications between two heterogeneous systems.

Figure 22: Gateways provide protocol translation between dissimilar systems at more than one OSI layer.

A gateway may consist of hardware, software, or a combination of the two, and it may provide translation at all or at only some of the different OSI layers, depending on the types of systems it connects.

There are a number of NetWare gateways that provide access to computer systems not based on the native NetWare/IPX protocol suite. NetWare for Macintosh is a software-based gateway that connects Macintosh computers to a PC-server-based NetWare network. NetWare for SAA is a gateway that enables NetWare users to transparently access SNA-based IBM hosts.

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